The present invention relates to viscosimeters and more particularly to such devices utilizing falling floats to measure the viscosity of fluids.
Viscosity measurement devices employing falling bodies which interact with the fluid under test are well known. Generally these consist of a transparent tube, closed at one end, and having a precision bore of constant internal diameter. The falling body, or float, having a diameter smaller than the inside diameter of said tube, describes an annular orifice of constant cross sectional area which is independent of the position of the float within the tube. At the open end of the tube the device may be equipped with a float release mechanism which is useful in holding the float in place until a measurement is to be taken. In order to take a reading, the tube is filled with a test fluid, the instrument is held in a vertical position, and the float is released and allowed to fall through the fluid. The float accelerates until it reaches a constant rate of speed, referred to as the terminal velocity. Time of descent is measured as it traverses a given distance, indicated by reference lines marked on the tube. From this information the viscosity of the fluid may be determined.
Since the rate of descent of the falling float is dependant on the shape as well as the size of the orifice formed between the tube and float, it is essential that the float remain centered during its descent. For this reason the outer tube is generally constructed with flats or beads, to insure concentricity. The use of such stabilizing flats may be avoided by maintaining only minimal clearance between the tube and the float, and by providing the float itself with an aperture through which the fluid may flow. Several devices employing such members are described in the prior art.
Gunn, deceased et al, U.S. Pat. No. 4,517,830 describes a viscosity measurement device in which blood viscosity may be determined immediately after withdrawal of the sample from the donor. The device consists of a hypodermic syringe in which a float member has been included such that the unit can operate as a falling float viscosimeter. The use of an apertured member is suggested in order to maintain concentricity during its descent. Although describing a float of similar structure to the present invention, Gunn does not suggest any means of extending the range of viscosities that the device is capable of measuring. Since the instrument is intended for the measurement of blood samples only, it is apparent that the float structure was selected solely to obviate the need for flats or beads on the inner surface of the syringe tube, which might otherwise interfere with the operation of the device. The Gunn device is not useful where values of viscosity vary widely and therefore no attempt was made to describe a generalized correlation between time of descent and absolute viscosity of the test fluid.
For applications requiring measurement of a range of viscosities rather than single values, it is desirable to have some means, whether experimental or theoretical, of correlating the absolute viscosity to the rate of descent of the falling body. In Heinz, German Patent No. 933172, a falling float viscosimeter is described in which the cylindrical float is provided with a capillary aperture. Minimal clearance is maintained between the float and the tube causing the fluid to flow only through the narrow, central aperture. Poisselle's equations, governing flow through capillaries, are used to determine the viscosity from the physical properties of the viscosimeter and the rate of descent of the float. This correlation however, only applies when the aperture is a capillary. The bore must also be sufficiently long such that entrance and exit effects would not introduce significant error. As a result, the range of viscosities that the device is capable of measuring is extremely limited.
A method of correlation more widely used for general viscometry work involves the determination of a viscosimeter constant. It has been shown that for a given viscosimeter, with a float and fluid of given densities, the viscosity of the fluid is directly proportional to the time of descent of the float falling at terminal velocity, provided measurement is limited to the Stokes region. The viscosimeter constant may be determined empirically, by measurement of a standard solution of known viscosity. It has also been suggested that the constant may be predicted theoretically from the dimensions of the tube and float. In the paper, A Falling-Ball Viscometer, by Roger Gilmont, Instruments and Control Systems, Sept. 1963, it was demonstrated that by applying equations governing flow through the annulus of a rotameter type flowmeter, the value of the viscosimeter constant could be expressed as a simple function of tube and float diameters, and the distance of travel at terminal velocity within the Stokes region. Application of flowmeter theory is justified, since there is essentially no difference between a float falling at constant velocity through stationary fluid, and fluid flowing past a stationary float.
In order for a viscosimeter to handle a wider range of viscosities it is most often necessary to alter the physical characteristics of either the float or the tube. Selecting a float material of greater density is one method by which range may be increased. However, where corrosive substances are to be measured, the choice of compatible float materials is limited Another alternative is to increase the viscosimeter constant by enlarging the orifice through which the fluid may travel. In the case of a solid float, such as a sphere, where flow is between the tube and float, this is accomplished by an increase in tube diameter, or by use of a smaller float. In either case an additional viscosimeter must be constructed, since a new tube is required.
In Eitzen et al, U.S. Pat. No. 2,431,378, a viscosimeter is described in which the falling body consists of a hollow cylindrical cup, open at the top, and having one or more apertures through which the displaced fluid may travel during measurement. Minimal clearance is maintained between the top lip of the cup and the outer tube to avoid wobbling. The cup-shaped structure serves to reduce the effective density of the float as compared with a solid float of comparable size, making the instrument especially desirable in applications requiring the measurement of fluids of extremely low viscosity. When designing a viscosimeter to measure fluids of higher viscosity, it is suggested that the float apertures may be enlarged to allow increased flow. However, because of the reduced effective density of the cup-like structure, a heavier cup is required to measure very viscous fluids. Although a new tube need not be constructed in order to measure the higher viscosity, correlation of absolute viscosity with time of descent is difficult. Due to the particular geometry of the float, enlargement of the apertures has little effect on the weight of the float, while the surface area perpendicular to flow is reduced. Both the effective density and the viscosimeter constant are affected as a result. The effect is the same when the weight of the float is increased to accommodate fluids of very high viscosity. This, combined with the complex structure of the float, makes the theoretical prediction of the factor relating viscosity with time of descent virtually impossible. No acceptable means of correlating rate of fall to viscosity are suggested in Eitzen, and since the device contains only a single float, it is incapable of measurement over a wide range. The complex nature of the float also makes manufacture costly, and limits the choice of materials from which it can be constructed.
Accordingly, the basic object of the present invention is to provide an improved viscosimeter of the falling float type, capable of continuous measurement over an extended range of viscosities.
Another object is to provide a viscosimeter in which the viscosity may be readily determined from time of the float's descent, without the need for frequent recalibration.
A further object is to provide these improved characteristics in a device which is accurate, reliable, corrosion resistant, and economical to produce.